Restore support of -Yexplicit-nulls compiler options for String and ::[?] values

Author: plokhotnyuk

build.sbt

@@ -241,6 +241,12 @@ lazy val `jsoniter-scala-next-tests` = crossProject(JVMPlatform, JSPlatform, Nat
   .settings(noPublishSettings)
   .settings(
     crossScalaVersions := Seq("3.7.3"),
+    scalacOptions ++= (CrossVersion.partialVersion(scalaVersion.value) match {
+      case Some((2, _)) => Seq()
+      case _ => Seq(
+        "-Yexplicit-nulls"
+      )
+    }),
     libraryDependencies ++= Seq(
       "org.scalatestplus" %%% "scalacheck-1-18" % "3.2.19.0" % Test,
       "org.scalatest" %%% "scalatest" % "3.2.19" % Test

jsoniter-scala-macros/shared/src/main/scala-3/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMaker.scala

@@ -1882,7 +1882,7 @@ private class JsonCodecMakerInstance(cfg: CodecMakerConfig)(using Quotes) {
     val tpe = types.head
     val implValueCodec = findImplicitValueCodec(tpe)
     if (implValueCodec.isDefined) '{ ${implValueCodec.get}.nullValue }
-    else if (tpe =:= stringTpe) Literal(NullConstant()).asExpr
+    else if (tpe =:= stringTpe) '{ null.asInstanceOf[String] }
     else if (tpe =:= booleanTpe) Literal(BooleanConstant(false)).asExpr
     else if (tpe =:= byteTpe) Literal(ByteConstant(0)).asExpr
     else if (tpe =:= shortTpe) Literal(ShortConstant(0)).asExpr
@@ -1913,7 +1913,7 @@ private class JsonCodecMakerInstance(cfg: CodecMakerConfig)(using Quotes) {
         typeArg1(tpe).asType match { case '[t1] => '{ IArray.unsafeFromArray(${genNewArray[t1](Expr(0))}) } }
       } else if (tpe <:< TypeRepr.of[mutable.BitSet]) '{ new mutable.BitSet }
       else if (tpe <:< TypeRepr.of[collection.BitSet]) withNullValueFor(tpe)('{ immutable.BitSet.empty })
-      else if (tpe <:< TypeRepr.of[::[?]]) Literal(NullConstant()).asExpr
+      else if (tpe <:< TypeRepr.of[::[?]]) '{ null.asInstanceOf[T] }
       else if (tpe <:< TypeRepr.of[List[?]] || tpe.typeSymbol == TypeRepr.of[Seq[?]].typeSymbol) '{ Nil }
       else if (tpe <:< TypeRepr.of[collection.SortedSet[?]] || tpe <:< TypeRepr.of[mutable.PriorityQueue[?]]) {
         val tpe1 = typeArg1(tpe)